Airway smooth muscle contraction can significantly alter the mechanical properties of the lung. Conversely, changes in lung volume have notable effects on the tone of contracted airways. As the airways are thought to be in a state of tonic contracture in vivo, the effect of changes in airway dimensions could play a significant role in the regulation of airway calibre. However, the properties of airway tissues under dynamic conditions have received little attention. The initial purpose of this study is to determine how continuous changes in muscle length affect the active force of tracheal and bronchial smooth muscle during tonic contracture with acetylcholine. Muscle strips will be suspended in a tissue bath so that length and force can be measured continuously during stretch and retraction. The relationship between active force during continuous stretch, and isometric muscle force will be measured under different conditions of length history. Active force will also be measured during continuous length cycling of the muscles at different rates over physiologic ranges of length. The second goal of this study is to determine mechanisms which affect the force developed by airway muscle during these dynamic maneuvers. Various factors affecting the kinetic properties of the muscle during tonic contracture will be investigated: 1) The effect of changes in muscle length; 2) The effect of the time course of the contracture; 3) The effect of the mode of activation by which contraction is produced. The influence of passive mechanisms will also be evaluated. The results of this study should be of importance to the understanding of the mechanics of contracted airways in vivo. An understanding of the mechanisms which govern airway properties during dynamic maneuvers should be of assistance in interpreting abnormalities in the mechanical behavior of the airways, such as those which have been shown to occur in asthmatics.